Most of the world's electricity is generated by combustion of a fossil fuel. Coal, oil (petroleum) and natural gas are all used for this purpose. From the standpoint of both economics and conservation, conversion from petroleum and natural gas to coal would be desirable. Known reserves of both petroleum and natural gas are dwindling, while abundant coal reserves remain, particularly in the United States.
Conversion from oil and natural gas to coal is hampered in many states of the United States by the high sulfur content in the coal supply. This sulfur, on combustion, forms sulfur oxides (SO.sub.x), which are atmospheric pollutants. Legal limits on SOx emissions have been established, and these promise to become more stringent in the future.
SO.sub.x emission specifications can be met by either desulfurizing the coal before combustion (pretreatment), by development of combustion techniques or additives that minimize SOx generation (concurrent treatment), or by scrubbing the stack gases (post treatment). Among these three emission control techniques, pretreatment has been proven to be the most economically and technically viable approach.
Physical cleaning methods presently being employed to remove sulfur from coal remove a large portion of the inorganic sulfur (mostly pyritic), but leave the organic sulfur untouched. Organic sulfur constitutes about one-half of the total sulfur in most high sulfur coals and, hence, economically viable chemical methods are needed to reduce organic sulfur content.
Supercritical extraction of coal has been previously described. "Supercritical extraction" herein refers to extraction of coal with one or more solvents at a temperature and pressure above the critical temperature and pressure, respectively (usually just above the critical temperature and pressure) of the solvent. For example, U.S. Pat. No. 3,988,238 describes desulfurization of coal by contacting the coal with water and optionally a co-solvent, which may be methanol, at a temperature either above or below the critical temperature. Amounts of co-solvent, when used, are small, and solvent-to-coal ratios are large (the water-to-coal ratio being at least 2:3 and preferably at least 1:1).
Prior attempts at supercritical extraction of coal have resulted in either the liquefaction of coal or substantial removal of organic coal components through conversion into liquid and gaseous components.